Recently, the cable-stayed bridges and the suspension bridges have become longer and larger. The cable-stayed bridges are advantageous in that they have high torsional rigidity of the bridge girder and excellent wind resistance performance against the crosswind. In addition, the cable-stayed bridge has a simple structure, and is advantageous in the cost of the construction. However, the length of the center span is about 1000 m at most in the cable-stayed bridges practically used. When the length of the center span of the cable-stayed bridge is large, the height of the tower in the cable-stayed bridge increases. Thus, the weight and the load to be supported by the tower increases, so that the size of the tower is enlarged. The load acting on the bridge girder in the bridge axial direction increases, too. Consequently, the structural advantageousness that the foundation structure of the tower becomes larger decreases.
Then, suspension bridges are usually adopted as long bridges where the center span is 1000 m or more. However, the height of the tower is also large in the suspension bridge, so that the foundation structure of the tower becomes larger. The cost of the construction of long and large main cables becomes high. Further, it is not easy to secure the wind resistance performance to the crosswind.
In case of cable-stayed bridges having large lengths of the center spans among conventional cable-stayed bridges, flat type one-box girder is often adopted as their bridge girders. In the conventional long suspension bridge, the above one-box girders are adopted as the bridge girders in some cases, while two-box girders having open space in the transverse direction for air ventilation are adopted in other cases.
Under the circumstances, in order to utilize the features of the cable-stayed bridge and those of the suspension bridge, there has been proposed the mixed structure named the cable-stayed suspension bridge in which the cable-stayed bridge and the suspension bridge are combined together. For example, two articles (Article Nos. 1 and 2) of “Static characteristics of the cable-stayed suspension bridge” (I-280) and “Examination of static properties of the cable-stayed bridge utilizing the suspension bridge in combination” (I-281) were published in the collected papers of the 45th Annual Science Lecture Meeting of Civil Engineering Association in Japan (September, 1990).
Further, an article (Article No. 3): “Trial designing of 900 m-span cable-stayed suspension bridge” was published in the collected papers of the 48th Annual Science Lecture Meeting of Civil Engineering Association in Japan (September, 1993). Furthermore, an article (Article No. 4): “Structural characteristics and economy of the long suspension type bridges” was published in Structural engineering collected papers (Vol. 41A) in Japan (March, 1995). Those articles examined the structure, the weight of the steel material, the rigidity, the wind resistance stability, and the economy with respect to cable-stayed suspension bridges having longer center spans, in which two main cables are stretched over plural towers and anchorage structures on opposite banks in a cable-stayed bridge having a long center span, while a suspension structure is adopted to suspend a central portion of the center span with the main cables and hanger ropes.
The structure mainly composed of the cable-stayed bridges are adopted in the cable-stayed suspension bridges described in these Article Nos. 1 to 4. For instance, in the cable-stayed suspension bridge of Article No. 3, the length of the center span is 1500 m, the length of the cable-stayed structure extending on the sides of the tower is 550 m, and the length of the suspension structure of the central portion in the center span is 400 m, while one-box girder is adopted as a bridge girder over the entire length.
On one hand, since the occupying ratio of the cable-stayed structure to the entire structure is large even in the cable-stayed suspension bridge, it is advantageous in the structure and the economy. However, it is hardly to say that such a bridge is a structure to be suitably applied to the long bridge having the length of the center span being 2000 m or more. For example, since the cable-stayed suspension bridge adopts the one-box girders as the bridge girder, the bridge girder in the central portion of the center span swings easily with crosswind. Therefore, it is difficult to raise a critical wind velocity at which fluttering begins to appear. Further, in case of the suspension structure in the central portion of the center span, the lower end portions of plural hanger ropes that extend downwardly from the main cables are connected to both sides in the transverse direction of the one-box girder. Therefore, the cable distance between the two main cables is large, so that the width of the top portion of the tower becomes larger, too and the tower becomes greater in size. Moreover, since the ratio of the length of the cable-stayed structure to the span length is large, the height of the tower is large for the span length, so that the towers become greater and the foundation structures for the towers become more bulky. Consequently, it is difficult to economically reduce the construction cost.
A first object of the present invention, which relates to a cable-stayed suspension bridge that adopts the cable-stayed structure and a suspension structure, is to increase the wind resistance of the bridge. A second object of the present invention is to attain the reduction in size of towers and foundation structures. A third object of the present invention is to reduce the diameter of main cables and attain the reduction in size of anchorage structures.